700 miles with O.S. Giken LSD

It's been slightly over 700 miles since I've had O.S. Giken TCD LSD installed along with solid diff bushings and M3 sub-frame bushings. VAC Motorsports built the diff for me.

During this period, I've had the chance to test things out during several track days and AutoX events. Here are my impressions:

1. The rear end of the car no longer feels wavy and disconnected. I get almost direct feedback from the rear wheels through the chassis, through the seat and into my spine. It's uncanny at first, but now after I've re-learned the car, I can say that this upgrade should have been the very first thing I did on my car.

2. The traction is crazy. I am now considering going with slightly narrower tires in the rear because there is so much more grip compared to the front.

3. Accelerating from stand still is now completely predictable. The rear is not longer waving back and forth unpredictably. The car goes in a straight line.

4. On the track and AutoX events, I no longer have to wait for the car to get completely out of the corner and settle down before I give it full power. The O.S. Giken does tremendous job keeping me stable mid corner, whether its under power or not! This is where the O.S. Giken shines compared to the rest of the diffs. You get lock during acceleration AND deceleration. This becomes really useful when trail braking into a corner.

Another difference is that if I change my mind mid corner and I let off the gas, I no longer get this unsettling of the rear end. Instead, I feel the diff unlocking and re-locking again in a split second as I lift off the throttle. It's kind of difficult to explain, but it works wonders for increasing the driver confidence in the car mid corner.

5. NVH. There is a faint whine during acceleration and deceleration. It is more prominent when the diff oil is cold. O.S. Giken make their own gear oil and on the bottle it is stated that optimal performance is achieved at 100C. The diff definitely quiets down once the diff fluid is warmer. The whine is a result of the solid diff bushings.

Today, I've spent a good portion of the day draining the fluid, lowering the diff housing and removing the diff cover. I wanted to inspect the condition of the oil and the ring gear after 700 miles of break in.

Here are some pics:

Notice how the fluid looks like graphite. This is after 700 miles of break in. The new fluid is clear with a faint yellowish tint. The viscosity is unchanged compared to new fluid, so this tells me that the diff fluid is managing the heat quite well even during track days.

On your point #2, your saying that the car has lost traction to the front...... Under steer?
Why would you take OUT traction to the rear with smaller tires, and instead just adjust your suspension to handle the new found grip?

do you have adjustable sway bars? Coil overs with adjustment? What is the suspension so we can dial it in with what you have?

the only thing reducing rear grip would achieve is less traction in the rear when trying to put down? the power, and more oversteer, which you'd see more than likely very easily at an easily achievable power level..?

Sway bars are THE LAST thing you do on a suspension. I have disproportionately larger rear wheels compared to the front. 285's on the back and only 255's on the front. Previously with the open diff, it used to make sense because it was very difficult to put all the power down. Now with the O.S.Giken TCD, there is a lot more traction so the car has more tendency to push and a it is a lot harder to get the rear to step out. It's that simple.

Yes, next I will probably get a proper coilover suspension and have that dialed in, but this is besides the point. I just wanted to share my impressions with O.S. Giken TCD diff, as the vast majority of people get either Wavetrac or Quaife, both of which behave differently.

Pretty penny! $3600 after core exchange + $900 install. This of course includes $200 in diff fluid alone and another $200 for all new hardware. I replaced every single bolt on the subframe and the diff as they are all aluminium and are one time use.

But its damn worth it! The diff shaved off about 2.5 - 3 seconds on AutoX on a 50 second course. I know this because I generally go to AutoX with the same groups of people and my times have been consistent.

On track, its difficult to say yet, because I still have not learned the new limits of the car, but I know I can accelerate sooner and finally be able to use trail braking on certain corners rather than applying the brakes.

Pretty penny! $3600 after core exchange + $900 install. This of course includes $200 in diff fluid alone and another $200 for all new hardware. I replaced every single bolt on the subframe and the diff as they are all aluminium and are one time use.

But its damn worth it! The diff shaved off about 2.5 - 3 seconds on AutoX on a 50 second course. I know this because I generally go to AutoX with the same groups of people and my times have been consistent.

On track, its difficult to say yet, because I still have not learned the new limits of the car, but I know I can accelerate sooner and finally be able to use trail braking on certain corners rather than applying the brakes.

ouch! That price point, killer.. But its good to hear that its working for you and you're enjoying it. Thats all that really matters.

I generally disagree with saying your swaybars are last in the list; and the subframe bolts and other related bolts to hold the differential in, are not aluminum

what special fluid is in that differential? I mean, BMW OEM limited slip oil is 135 for 90-130 with slip additive, what special requirements does it have?

1. Yes, they are aluminium. I ordered them, I looked at them, I licked them. Yes, aluminium.
2. O.S. Giken Diff fluid is actually 80-250 and it is specifically designed for their LSDs which have 20 clutch plates inside:

ouch! That price point, killer.. But its good to hear that its working for you and you're enjoying it. Thats all that really matters.

True, it is very expensive. The TCD diff is the most expensive diff you can get for our cars. The fact that I had welded ring gear did not help either. That adds extra $700 for machining and assembly compared to bolted ring gears.

I'm pretty sure those bolts are steel. The differential mounting bolts have 10.8 stamped in the head which is somewhere between grade 5 and grade 8 SAE bolt strength if I recall correctly. They have to be steel to achieve that strength. Aluminum is simply not strong enough to be used for such critical points. They are galvanized, however.

im absolutely positive that the subframe bolts are not aluminum. They do have a nice sheen of silver on them however

i cant count how many subframes i have dropped on BMW's, but its a moot point to argue about not having one aluminum bolt on that rear suspension, and only having aluminum bolts where aluminum/magnesium would contact each other

I also find that a very odd viscosity. anyone else think an 80-250 is a very extreme spread?